1. Field of the Invention
The present invention relates to fixing devices which fix a toner image on a recording material in image forming apparatuses using, for example, an electrophotographic method.
2. Background Art
In image forming apparatuses, such as copying machines and printers, which use an electrophotographic method, a photosensitive body (photosensitive drum) formed in the shape of, for example, a drum is uniformly charged and the photosensitive drum is scanned and exposed with light controlled on the basis of image information so that an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image is then turned into a visual image (toner image) with toner. Then, the toner image is directly transferred to a recording material from the photosensitive drum, or the toner image is primarily transferred to an intermediate transfer medium and secondary transferred to a recording material from the intermediate transfer medium. Thereafter, the toner image is fixed on the recording material by a fixing device.
The fixing device used for such image forming apparatuses is composed of, for example, a fixing roller in which a heating source is disposed within a cylindrical core bar, and which is formed such that a heat-resistant elastic layer and a release layer on an outer peripheral surface of the elastic layer are laminated on the core bar; and a pressing roller which is disposed in pressure contact with the fixing roller parallel thereto and formed such that a heat-resistant elastic layer, and a release layer, made of a heat-resistant resin film or a heat-resistant rubber film, on an outer peripheral surface of the elastic layer are laminated on a core bar. Also, a recording material having an unfixed toner image carried thereon is caused to pass between the fixing roller and the pressing roller so that heating and pressing is performed on the unfixed toner image, thereby fixing the toner image on the recording material. Such a fixing device is called a two-roller fixing method and it has generally been widely used.
Meanwhile, in such a conventional fixing device using fixing rollers like the two-roller fixing method, the fixing rollers have their own large heat capacity. Therefore, there is a problem in that even when supply of power to the fixing devices is started simultaneously when a power source of the image forming apparatus is turned on, considerable time is required until the fixing rollers rise in temperature from room temperature to a fixable temperature (warm-up). Because of the characteristics of the fixing rollers that quick start is difficult as such, when the image forming apparatus is in a standby state, it is also necessary to keep the temperature of the fixing rollers always constant to prepare for the start of the image forming apparatus. Therefore, there is also a problem in that electric power consumption of the fixing device is large.
In order to solve such problems, fixing devices using an endless fixing belt member have been developed instead of the configuration using the fixing roller. Since the fixing belt member has its base member made of a film-like heat-resistant resin or the like, it has advantages in that the heat capacity is small and the warm-up can be performed in a short time, as compared to a roller-shaped member such as the fixing rollers. Moreover, since the quick start is easy, the electric power consumption of an image forming apparatus in a standby state can also be reduced.
As an example of conventional techniques related to the fixing devices using the fixing belt member, the following technique is suggested (for example, see JP-A-2003-223064). According to this technique, the fixing device is configured such that The fixing device also is composed of a heating film (fixing belt member) which has a halogen heater disposed in an inner space and is rotatably supported by a support member, and a pressing roller member disposed in pressure contact with the heating film to form a fixing nip part so as to drive the heating film so that the heating film follows the pressing roller member. Infrared rays emitted from the halogen heater are converged on the fixing nip part to heat the heating film in the fixing nip part, thereby fixing a toner image on a recording material passing through the fixing nip part on demand.
Meanwhile, even in the fixing device using the above-described fixing belt member, the market needs to further shorten the warm-up time have been increasing. In order to meet such needs, it is necessary to further reduce the heat capacity of the fixing belt member. For this purpose, it is also necessary to reduce the diameter of the fixing belt member.
However, if the diameter of the fixing belt member is made small, the width of the nip part which applies heat to a recording paper never fails to become small in terms of its structure. Therefore, it is difficult to supply the amount of heat enough to melt a toner image to a recording paper while the recording paper passes through the nip part for a slight time. As a result, there is a problem in that the processing speed must be reduced due to the requirements for ensuring the amount of heat in the nip part, and thus it is difficult to shorten the warm-up time in high-speed image forming apparatuses.